This invention relates generally to fuel systems for internal combustion engines and more particularly to carburetors having an idle speed adjustment screw.
Carburetors are generally utilized to produce and control the mixture of fuel and air delivered to an internal combustion engine. A carburetor having a main body with a mixing passage generally includes a throttle valve that is mounted on a shaft rotatable by a lever. The throttle valve is adjustable to various positions including an engine idle speed position and a wide-open throttle position. A spring is usually utilized to yieldably bias the throttle lever and the throttle valve to its idle position. To permit adjustment of the idle position, an idle speed adjustment screw of steel is received in the carburetor body and has a conical tip on its free end. The tip generally provides an adjustable stop that is engaged by an arm on the throttle shaft when the throttle valve is in the idle position. Rotation of the idle speed adjustment screw changes the location of the tip relative to the arm thereby changing the position at which the arm bears on the tip and thus the rotary position of the arm and throttle valve. In this manner, the idle position of the throttle valve is adjusted thereby modifying the idle speed of the internal combustion engine.
Typical idle speed adjustment screws are metallic and include a compression spring associated therewith to provide a generally axial force on the idle speed adjustment screw such that it does not change position during the operation of an internal combustion engine.
Idle speed adjustment screws are commonly adjusted during manufacture for various engines and idle speed configurations. A problem associated with utilizing a metallic idle speed adjustment screw is that the spring can lose its ability to provide sufficient torsional resistance if overcompressed resulting in undesirable movement of an idle speed adjustment screw. Also, after repeated adjustments to the idle speed adjustment screw, a metallic compression spring may lose its ability to provide sufficient torsional resistance for the screw to eliminate undesirable movement.
A carburetor idle speed adjustment screw is formed of a polymeric material such that its shank is deformed by engagement in a threaded bore and retains the idle speed screw in its adjusted position within the threaded bore, even after repeated adjustments. The carburetor has a main body with a fuel and air mixing passage in which a throttle valve is rotatably received to adjust the quantity of a fuel and air mixture delivered to an engine. A support member is carried by the main body and includes a threaded bore in which the shank of the idle speed screw is threaded to adjust and control an idle position of the throttle valve.
A polymeric idle speed adjustment screw facilitates adjustment of the engine idle speed by a user by retaining its adjusted position even after multiple adjustments. Further, the polymeric idle speed adjustment screw eliminates the need for a compression spring, therefore reducing the number of parts associated with the carburetor and thereby reducing the cost to manufacture and assemble the carburetor.
Objects, features and advantages of this invention include providing a carburetor which facilitates adjustment of the idle speed by an end user or operator, eliminates the need for a compression spring associated with a metal idle speed adjustment screw, reduces the number of parts in the carburetor, provides an idle speed adjustment screw that is capable of multiple adjustments without a loss of retention in its adjusted position within a threaded bore, is of relatively simple design and economical manufacture and assembly, and in service has a long useful life.